The present invention relates in general to communication systems, and is particularly directed to a method and arrangement for improving link bandwidth efficiency and power usage of a satellite communication system, by replacing the modulation and encoding mechanisms employed by the head end of the system with enhanced modulation and encoding schemes, and augmenting each receiver site with a front end link adapter interface that demodulates the replacement modulation and coding, and reconverts the communication protocol of the enhanced link into the original protocol for demodulation by a non-upgraded receiver.
The substantially increased levels of link performance that have evolved from ongoing improvements in communication signal processing technology will eventually drive communication system service providers to the conclusion that at least some if not all of their equipment must be replaced or upgraded. Where the number of components of an existing system infrastructure is relatively large, as may occur in the case of a satellite broadcast system, for example, conducting a wholesale replacement of all of the existing receivers with newer, higher performance components is effectively cost-prohibitive. In the satellite communication system illustrated diagrammatically in FIG. 1, broadcast signals are uplinked from a head end terminal site 10 through a satellite 20 and are typically downlinked to hundreds or thousands of remote earth terminal sites 30.
In accordance with the present invention, this replacement cost problem is successfully addressed by substituting the modulation and encoding mechanisms employed by the head end of a communication system (such as a satellite-linked DATS system) with a combined, enhanced modulation and encoding scheme that is effective to provide significantly improved link performance in terms of bandwidth efficiency and/or power usage. In addition, each downlinked receiver site is retrofitted with a front end link adapter interface that is operative to demodulate the link-efficient, replacement modulation and coding to baseband, and then reconvert the demodulated baseband signals back into the original modulation and encoding protocol used by the existing non-upgraded receiver, so that the in-place receiver may demodulate the signal.
To this end, at the head end terminal, a plurality of digitized TDM baseband channels such as those supplied from network studio sites, that are typically but need not be multiplexed into an aggregate baseband serial bit stream. This aggregate bit stream is then converted into packetized format by an MPEG (motion picture expert group) II protocol converter. The MPEG-converted packetized data is then modulated for transport over the link by a high performance modulation and encoding mechanism. In a preferred embodiment, this mechanism comprises a digital video broadcast (DVB) modulator that performs QPSK modulation and enhanced FEC encoding in the form of concatenated Viterbi/Reed-Solomon encoding.
By high performance or enhanced modulation and encoding is meant that it provides substantial reduction in link bandwidth usage and/or power by the modulator, so that for the same or improved BER, the bandwidth efficiency of the link is increased, and/or at a lower power requirement. Because of the two to one improvement provided by QPSK modulation, and the improved BER afforded by the concatenated Viterbi/Reed-Solomon encoding, DVB modulation provides a very practical mechanism for realizing these objectives. In addition, DVB modulation provides for operation at data rates up to 60 Mbps, so that transmission rates well in excess of a conventional DATS system are available.
The QPSK-modulated and FEC encoded signal is transmitted by existing up-conversion and transmission equipment over an uplink RF channel to the satellite transponder. The satellite transponder broadcasts the DVB modulated signal over its downlink channel to a plurality of remote earth terminal sites. In order to receive the DVB signal, a respective remote terminal is augmented to incorporate a front end link adapter interface that is installed between its wideband RF receiver and the in-place DATS demodulator. A first embodiment of the link adapter interface includes a DVB demodulator that performs QPSK demodulation and Viterbi/Reed-Solomon decoding of the received signal, and outputs a baseband signal having the same MPEG II packetized format as applied to the DVB modulator at the head end.
To make this signal compatible with the existing DATS receiver equipment, it is reformatted and remodulated by means of an MPEG-to-DATS protocol converter, which strips off the transport overhead and reassembles the packets into an aggregate multi-T1 baseband digital bit stream, that corresponds to the input to the MPEG II protocol converter in the upgraded head end equipment. The aggregate multi-T1 baseband digital bit stream is then coupled to an FEC encoder which performs the same FEC encoding mechanism that had been previously performed by the DATS baseband processing unit of the replaced head end. The output of the FEC encoder is then remodulated back into DATS BPSK to provide the DATS demodulator of the existing receiver equipment with exactly the same type of signal it expects to receive from the wideband RF amplifier.
In addition to being coupled to an FEC encoder, the aggregate multi-T1 baseband digital bit stream output from the MPEG II protocol converter may also be coupled directly to the baseband processing section of a DATS receiver that is configured to accept a baseband input.
A second embodiment of the link adapter interface is configured to accommodate the situation where the network service provider is in the process of replacing the conventional DATS head end equipment with upgraded DVB head end equipment. The second embodiment is configured to continue to provide a default connection from the RF receiver output to the DATS receiver equipment. However, when the link adapter detects that the upgraded head end equipment is operational, it automatically switches in the functionality of the augmented receiver of the first embodiment.
A third embodiment of the link adapter is configured to accommodate the replacement of the DATS receiver with a DVB receiver. In this third embodiment, a demultiplexer is installed at the output of the DVB demodulator and has a first output ported to the MPEG-to-DATS protocol converter and a second output ported directly to a DVB-compatible receiver. The steering path through the demultiplexer provides either a remodulation DATS path, as in the case of a standard DATS receiver, or a path for direct DVB baseband processing of the output of the DVB demodulator by a DVBxe2x80x94MPEG II compatible receiver.